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The invention relates generally to measurement of optimal lung volume during high frequency oscillatory ventilation of humans or other mammals. More particularly, this invention relates to the utilization of pressure measurements at the proximal and distal ends of an HFOV patient endotracheal tube as a proxy or indicator for optimal lung volume.
Frequently, a sick patient must be assisted in breathing by a ventilator. This patient may be human, or a non-human mammal. During conventional mechanical ventilation (CMV), the lung is inflated with a distending pressure called positive end inspiratory pressure (PIP). During HFOV, the lung is inflated with a continuous distending pressure called mean airway pressure ({overscore (P)}aw). This mean airway pressure {overscore (P)}aw is superimposed with oscillating pressure variations.
In a diseased lung, some air sacs may collapse, preventing gas from entering or leaving and thereby preventing gas exchange through those air sacs. Because a fewer number of air sacs are available for gas exchange, the patient must be ventilated with a higher concentration of oxygen than normal to enable his or he remaining open air sacs to provide adequate blood oxygenation. While a high oxygen concentration is required to provide adequate blood oxygenation and keep the patient alive, it is also toxic.
During inflation of the lung with increasing PIP or {overscore (P)}aw, the pressure increases and the collapsed air sacs of the lung begin to open, allowing them to once again take part in gas exchange. The pressure at which the air sacs begin to open is called the critical opening pressure. Air sacs opened because of ventilator inflation pressure are said to be recruited. As the number of recruited air sacs increases, the amount of oxygen that diffuses into the arterial blood also increases. This is reflected as an increase in blood oxygen saturation level, as may be non-invasively measured by pulse oximetry or directly with an arterial blood gas measurement. The increase in oxygen in the blood enables the caregiver to lower the inspired oxygen concentration toward less toxic levels. For these reasons, it is generally beneficial to recruit as many air sacs as possible in a patient undergoing ventilation.
In HFOV, when an increase in the ventilator mean airway pressure fails to improve the oxygen saturation level of the blood, the lung is considered to be recruited. If additional pressure is added to a stable lung, the patient runs the risk of experiencing overinflation. Overinflation significantly increases the chances for lesions to form in the lung tissue. Such lesions can allow air to leak into the space between the lungs and the chest wall, and can be lethal. In addition, overdistention also adversely affects pulmonary bloodflow.
If the lung has been pressurized to the point of overinflation during recruitment, the pressure can be reduced to alleviate overdistention. However, due to the elasticity of the lungs, which causes a nonlinear pressure/volume relationship which is different for inhalation than exhalation, the window between overdistention and lung collapse can be small. If the pressure is weaned too low, some alveoli will start to collapse. This pressure at which the air sacs begin to derecruit is called the critical closing pressure. Consequently, to prevent overinflation in a given patient, ventilator pressures must be reduced to a lower level to find the safest pressure with the air sacs recruited. Unfortunately, it is quite easy to bypass the window between overdistention and lung collapse. When a significant number of alveoli collapse, the patient""s blood oxygen can fall to dangerously low levels. As can be seen, this trial and error method is risky for the ventilated patient.
Underinflation of the lung creates another set of physical problems. If the lung is underinflated, diseased lung tissue may be derecruited, causing a condition called atelectasis. That is, diseased air sacs that took part in gas exchange when the lung was properly inflated (i.e., air sacs that had been recruited) will no longer do so if the inflation pressure is too low. Those air sacs will close again, and no gas exchange will take place through them, inhibiting the patient""s ability to absorb oxygen and jettison carbon dioxide. Underinflation thereby causes atelectasis, which may be a life-threatening condition. Finally, underinflation can result in the release of chemicals in the lung tissue that induce biochemical lung injury.
Given the dangers of overinflation and underinflation, the pressure output of a ventilator must be high enough to prevent underinflation, and low enough to prevent overinflation. In the case of HFOV, the mean airway pressure {overscore (P)}aw must fall in this rather narrow range between underinflation and overinflation.
During HFOV operation, it is desirable to achieve maximal lung recruitment and minimal overdistention. Unfortunately, {overscore (P)}aw is not, by itself, an indicator that can be used to determine when this point has been reached. There thus is a need for a relatively simple yet accurate method of determining the optimal lung volume for lung injury patients. A need also exists for a device that can be easily and safely adjusted to the optimal lung volume of the patient.
In a first aspect of the invention a method for determining the optimal lung volume for a patient on high frequency oscillatory ventilation includes the following steps. Initially, the peak-to-peak oscillatory pressures are measured in the proximal and distal ends of an endotracheal tube positioned within the patient. The oscillatory pressure ratio is then calculated from the peak-to-peak oscillatory pressures in the proximal and distal ends of the endotracheal tube. The mean airway pressure is altered and the oscillatory pressure ratio is recalculated at the altered mean airway pressure. The mean airway pressure is subsequently altered and the oscillatory pressure ratio is recalculated until the oscillatory pressure ratio is at or near its minimum value.
In a second aspect of the invention a method for determining the maximum lung compliance of a patient on high frequency oscillatory ventilation includes the following steps. Initially, the patient is provided with high frequency oscillatory ventilation at an initial mean airway pressure. Next, the peak-to-peak oscillatory pressures in the proximal and distal ends of an endotracheal tube positioned within the patient are measured. Based on these measurements, the oscillatory pressure ratio is calculated from the peak-to-peak oscillatory pressures in the proximal and distal ends of the endotracheal tube. The mean airway pressure provided to the patient is then increased. The oscillatory pressure ratio at the increased mean airway pressure is then calculated. The mean airway pressure is subsequently increased and the oscillatory pressure ratio recalculated until the oscillatory pressure ratio increases.
In a third aspect of the invention, the method according to the second aspect wherein the step of increasing the mean airway pressure is replaced by the step of decreasing the mean airway pressure.
In yet a forth aspect of the invention, a device for determining the maximum lung compliance of a patient on high frequency oscillatory ventilation includes a first pressure sensor at a proximal end of an endotracheal tube in the patient and a second pressure sensor at the distal end of the endotracheal tube. The device also includes means for calculating the oscillatory pressure ratio based on measurements obtained from the first and second pressure sensors. A mean airway pressure controller is provided for altering the mean airway pressure delivered to the patient. The device also includes a display for displaying the oscillatory pressure ratio.
It is an object of the invention to provide a method and device for determining the optimum mean airway pressure for maximal lung recruitment and minimal overdistention. The method and device is useful on patients with lung damage or lung disease. The method and device will also work with patients with healthy lungs. Generally, the method and device are useful in adult and pediatric patients with ARDS or acute lung injury. The method and device are particularly useful in newborn infants suffering from idiopathic respiratory distress syndrome (IRDS).